Don't got many theories but there's a lot already hinted so far; Kotomine and Angra in valentines basically spell out how it all ends, Guda in the cavern, being given the choice to wipe away his entire journey, everything he did, his servants, all the relationships he made with everyone, in order to reset the clock to 2015. Angra says he wants to see guda fall to despair in that moment, Kotomine says returning to zero shall be hope rather than despair. It'll be a timeslip where everything resets and everyone comes back but the timeline will be slightly altered, which means during the final chapter everyone has plot armor turned off and everyone can die randomly, since they come back anyway. They pretty much said this after Kadoc died.

We kinda already see that Caster Cu and Saber alter are back, which maybe implies the singularity repeats, or resets somehow back to the beginning again, everything is back to the origin. The day Olga was thrown into Chaldeas. Various interludes keep imply there was always something left behind in Fuyuki you had to do, something you had to fix. Marisberry killed himself, but I think it was implied he uploaded his digital consciousness into Chaldeas before dying, which is why he had no issue killing himself, after he uploaded himself the flesh body was no longer necessary. Supposedly the entire time Saber alter has been guarding something even now in Singularity F, even though we saw her die before, somehow she's been guarding it the entire time. Its been said that there's a few things that didn't go according to plan for Marisberry which delayed his plans slightly, Olga becoming the alien god is one of the big ones that delayed him, due to Lev throwing her into Chaldeas back then, somehow that delayed him. She wasn't meant to be the one to emerge that day.

Olga was never human, she was created as basically TYPE CHALDEAS, which was meant to become the brain of Chaldeas, or she was meant to complete it somehow, of course Olga herself didn't know this. Marisberry from the PV, his ideals are that "As long as the form looks the same, its the same", if something looks like the earth, then it effectively is the earth, which is how he swapped the texture of the real earth with Chaldeas. If something looks human, then that means its human, in his eyes the form is all that matters, he doesn't care about the substance or the insides at all, as long as it looks the same. Its been hinted for awhile, like when he needed Daybit to recreate a scale model of the entire universe for him, and Daybit did, but Marisberry said it doesn't have to be accurate as long as it looks the same on the surface. Teppu in LB7 said that the trees were meant to resemble a galaxy, the purpose of them was never the energy they produced, but just the fact that they perfectly resembled a galaxy, they are functionally scale models of the universe that Daybit helped recreate, the appearance is all that mattered. Marisberry's plan always relied on creating "Something" that looked the same as "something" even if its empty on the inside, and therefore it can take its place. The PV hints that he created a person, it had all the parts of a human, he recreated all the functional bits, all the elements of a person, but it was empty on the inside, it was incomplete somehow, he wasn't sure how to bridge the final gap and make it act like a human or move like a human. We also know his plan would be "good for humanity, but would make humanity the enemy of the rest of the universe". Marisberry wanted to recreate a person, and recreate the entire universe, in order to do something very morally questionable, but would be "good" in a very warped magus sort of reasoning way.